This invention relates to a method for preparing malonate group-containing acrylate monomers.
Michael reactions in which base-catalyzed addition of activated methylene to polarized double bond are utilized in the crossling reaction of resinous compositions for coating use. These compositions are advantageous in that the Michael reaction gives a chemically stable crosslinking bond without forming any reaction by-product.
Typical examples of activated methylene compounds used in the Michael reaction are acetoacetates, cyanoacetates and malonates. For use in the crosslinking reaction of resins or polymers, the Michael donor must be a compound or polymer having a plurality of activated methylene groups in the molecule. Acrylic monomers having an acetoacetoxy group can be synthesized relatively easily by reacting diketene with a hydroxylalkyl acrylate or methacrylate such as 2-hydroxyethyl acrylate (HEA) or 2-hydroxylethyl methacrylate (HEMA). Activated methylene-containing acrylic polymers used in the prior art are either polymers of acetoacetylated hydroxyalkyl (meth)acrylate monomers or acetoacetylated acrylic polyols produced by reacting diketene with an acrylic polyol polymer.
Because malonic acid is a dibasic acid, malonate groups can be incorporated into a polyester chain to produce a polymeric Michael donor. U.S. Pat. No. 4,602,061 discloses a liquid two-component coating composition comprising (a) an oligomeric or polymeric malonate compound such as polyurethanes, polyesters, polyacrylates, epoxy resins, polyamids or polyvinyl resins containing malonate groups, either in the main chain or the side chain or in both, (b) an .alpha., .beta.-ethylenically unsaturated carbonyl compound and (c) a basic catalyst.
The malonate group-containing polyacrylate resin disclosed therein is a reaction products of the above-mentioned malonate polyester resin first with a diisocyanate and then with HEA or HEMA.
JP-A-03206012 discloses a series of acrylic monomers in which one of carboxyl groups of an aliphatic dicarboxylic acid such as malonic, succinic or glutaric acid is esterified with an alkanol while the other carboxylic acid is esterified with an alkanol. These monomers are used for the production of odorless, low irritating and low water absorption polymeric dental materials. Although 2-(methoxymalonyloxy)ethyl methacrylate otherwise called 2-methacryloyloxyethyl methyl malonate is named, its production method is not specifically disclosed. Instead, it is taught that the malonate monomer can be produced in the same manner as the corresponding succinate monomer which is produced by reacting succininc anhydride with HEMA and then reacting the resulting half ester with methanol. Because malonic anhydride does not occur, this method is not applicable to the malonate monomer.
It has been discovered that acrylic polymers having malonate group-containing pendant groups exhibit higher weatherability than the corresponding polymers having actoacetate-terminated pendant groups when used for coating purposes. This is because the terminal structure of activated methyle is a stable ester linkage.
A need exists, therefore, for a method for preparing a malonate group-containing acrylate monomer which is suited for large scale production of the malonate monomer efficiently starting from easily available materials.